Rollup merge of rust-lang#106172 - estebank:suggest-impl-trait, r=compiler-errors

Suggest `impl Iterator` when possible for `_` return type

Address rust-lang#106096.

Author: matthiaskrgr

core/src/iter/traits/iterator.rs

@@ -66,6 +66,7 @@ fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub trait Iterator {
     /// The type of the elements being iterated over.
+    #[rustc_diagnostic_item = "IteratorItem"]
     #[stable(feature = "rust1", since = "1.0.0")]
     type Item;